1. Field of the Invention
The invention relates to an oil mist separator for an internal combustion engine.
2. Description of the Related Art
It is known that when so-called blow-by gas, containing unburned fuel, that leaks through the gap between a piston and a cylinder into a crankcase is mixed with the oil in the crankcase, so-called sludge is produced, which significantly accelerates the deterioration of the oil. The main components of sludge are olefin (hydrocarbon) in the oil, and NOx and water in the blow-by gas, and such main components react with the help of heat and acid to produce a sludge precursor and a sludge binder, which in turn produce the sludge. Sludge looks to be a mud-like substance.
A positive crankcase ventilation (PCV) system is available that, in order to suppress deterioration of oil, introduces the blow-by gas in the crankcase into the inlet system to combust the unburned fuel in the blow-by gas (see Japanese Patent Application Publication No. 2003-322052 (JP-A-2003-322052)).
Because the blow-by gas in the crankcase contains an oil component, it is common that an oil mist separator is provided in the path along which the blow-by gas is introduced. In general, the oil mist separator includes a plurality of baffle plates therein. While the introduced gas passes through a gas passage that is defined by the baffle plates, the gas hits the baffle plates, the oil is thus separated from the gas, and the separated oil is returned into the crankcase.
However, there is a problem that such an oil mist separator does not effect sufficient separation of oil from the gas. Therefore, in Japanese Utility Model Publication No. 1-15852, a technology is described for compensating the insufficient capability of the baffle plates in separating the oil component, by providing a porous filter made of foam metal in the oil mist separator.
However, when a porous filter is provided in the oil mist separator, there is a possibility that sludge is produced and therefore clogging of the porous filter occurs. In particular, because the oil mist separator is exposed to the air, formation of condensed water easily occurs therein. Because the condensed water and NOx in the gas produce nitric acid, sludge is easily produced. There is a problem that when the porous filter clogs, the flow of gas is obstructed and the capability that the oil mist separator originally has is deteriorated.